Lateral stacking disorder in hard-sphere crystals (AGGREGATION CH 2)

Abstract

We demonstrate that randomly stacked hard-sphere colloidal crystals do not merely consist of random sequences of hexagonally close-packed layers, but also of islands within the hexagonal layers with different lateral positions A, B and C. The existence of such in-plane stacking disorder was suggested by a recent observation of lateral broadening of the Bragg scattering rods in micro radian X-ray diffraction and is further confirmed here by real-space confocal microscopy in two hard-sphere colloidal systems with different sedimentation Peclet numbers. The transition from one lateral position to the other either occurs discontinuously through line-defects or continuously in a bridging area through a lattice deformation and results in transitions between hexagonally close packed and face centered cubic structures. The subsequent chapter is dedicated to the continuous transitions. In this chapter the discontinuous linedefects are characterized, up to their 3-D structure. The chance ζ to find another line-defect above a line-defect in the layer below turns out to be close to 1/2 – independent of relative gravity – which can be explained by the two different stacking options above a defect. The stacking of a few sets of several line-defects situated on top of each other turns out to be predominantly FCClike.